Perseus constellation provides plenty to see


By Tom Burns - Stargazing



Is it any wonder that many avid stargazers detest daylight saving time? When DST begins on March 14, we can’t get a decent view of the night and still get to bed at a reasonable hour.

March 14! I’m not much of a conspiracy theorist, but does anybody remember when DST was just a summer thing? Over the years, eastern standard time has shrunk to just a few months. EST is now the exception, not the rule. “Standard” time isn’t standard anymore, and it’s about time that we stopped pretending that it is.

God bless EST. For 12 more nights at least, by 7:30, blessed darkness has fallen.

At that time, the constellation Perseus is setting beautifully in the northwest about halfway up to the top of the sky (or “zenith,” as we astronerds say). The brighter stars of the ancient Greek hero form an upside-down “Y.”

Here, my well-rested stargazing friends, there is much to see.

Start from the brightest star of the constellation, located at the center of the “Y.” The star Mirfak forms the chest of the hero.

The old Arabic astronomers, who mapped the sky with great precision, thought differently. They named it Mirfak, a shortened version of Mirfak al Thurayya, the “Elbow Nearest the Many Little Ones.”

“Little Ones” there are aplenty near Mirfak. Look at the area with binoculars, and you will discover the dozens of stars in the “Perseus Association,” which provides one of the best binocular views in the sky.

Mirfak is relatively distant from Earth for a naked-eye star at something like 620 light-years away. To be so bright, it must be powerful. The star produces 4,000 times the energy output of our puny daytime star, the sun.

Perseus and neighboring Cassiopeia are renowned for their abundance of open star clusters, loose collections of young stars that were all born out of the same cloud of mostly hydrogen gas.

Up and to the right from Mirfak is M34, visible in a small fuzzy patch in binoculars. In a small telescope, it will resolve into a few dozen stars. Like all star clusters, the stars of M34 are gravitationally bound to each other and thus travel together in space.

The area’s showpiece is probably the Double Cluster, which is on the border between Cassiopeia and Perseus.

The Double Cluster is easily visible to the unaided eye from a dark, rural location. This time of year, look above the “W” shape of the constellation Cassiopeia.

The ancients were well aware of this strangely aberrant fuzzy patch intruding upon a sky filled with point-like stars. As early as 150 BCE, Hipparchus, the great stellar cartographer, cataloged the object as a single patch of light.

It took two millennia and advances in telescope technology for English astronomer William Herschel to recognize that the Double Cluster was indeed double.

NGC 869 and NGC 884, as the two clusters are officially called, are close enough to each other that they fit nicely in the same medium-power telescope field.

Taken together, the two clusters have a mass of 6,500 stars like our sun. In a small telescope, the two adjacent fuzzy patches resolve into scores of stars. They look like two piles of gems stacked up next to each other.

At only a few hundred light-years away from each other, NGC 869 and NGC 884 appear to be gravitationally bound. As a result, they travel together as they move at about 24 miles/second in our direction.

But not to worry. They are quite far away at 7,500 light-years. (One light-year is equal to about 5.9 trillion miles.) By the time they reach our environs in about 9.5 billion years, their loosely bound stars will have scattered into the galaxy.

Long before, the sun will have collapsed to a white-dwarf star. Our day star will no longer produce its bountiful heat and light, and Earth will be a cold, dead place.

The stars of the two clusters are quite young as stars go. They emerged from the cloud that gave them birth only 13 or so million years ago. By comparison, our middle-aged sun is about five billion years old.

As beautiful as the Double Cluster is, the real “star” of Perseus is Algol, the Demon’s Head or Demon Star.

Find it by looking to the right and slightly down from Mirfak. Observe it over the course of several nights, and you will notice that it varies distinctly in its brightness.

Algol has a long and storied history. From early times, it has represented the head or the winking eye of the snake-haired Medusa, which Perseus holds, newly severed, in his hand. One look at her face turned the unfortunate viewer to stone, so don’t look at Algol too long.

Algol winks because it is in a class of stars that vary in their brightness. Stars brighten and dim for various reasons, but Algol changes because it is, in fact, not one star at all. In this three-star system, one of the unseen, dimmer stars orbits the bright one we see. As the dimmer companion passes in front of the bright star, the brighter one gets dimmer. Thus, every 2.85 days, the brighter star dims for about 10 hours – a slow-motion wink.

Italian astronomer Geminiano Montanari was the first to record Algol’s variability in 1667, but the ancients must have noticed the wink. Why else would they put the starry version of the hideous Gorgon’s head in Perseus’s outstretched hand?

How did such a great hero end up clutching such a monstrosity? Read on, curious reader.

King Acrisius should have been a happy man. He ruled the bountiful land of Argos. His daughter Danae charmed everyone with her intelligence and beauty. But Acrisius was driven nearly insane by a prophecy that foretold his death by a son, yet unborn, of his innocent daughter.

To prevent Danae from marrying, the king locked her in a tower. He vowed that the all-too-human eyes of potential suitors would never again gaze upon her.

The unlucky king didn’t figure on immortal eyes, however. Zeus, king of all the gods, easily saw through the tower walls.

“Hubba, hubba,” he said in ancient Greek and turned himself into a shower of gold coins. He then rained himself down on Danae’s lap through a skylight.

Nine months later, Perseus was born. In anger and fear, Acrisius set his daughter and grandson adrift into the sea in a wooden chest.

This was, of course, terrible parenting, but Danae’s desperate pleading must have sounded false. “It was the coins, Dad, the COINS, honest.”

The chest came to ground at Seriphos, an island ruled by the vicious Polydectes. There Perseus grew to young manhood.

All the while, Polydectes kept a lustful eye on Danae. After his wife died, Polydectes asked the comely woman to marry him.

She said no. Polydectes, true to his nature, imprisoned her in his palace and turned her into a love slave.

Polydectes knew that he had to rid himself of Perseus, so he gave him an impossible task — to bring home the head of Medusa, the Gorgon. You were wondering perhaps when we would get to the Gorgon. Well, here she is.

The Gorgons were three sisters with live snakes for hair and a single eye and tooth, which they had to share. Medusa was so ugly that a single glance at her face turned the unfortunate viewer to stone.

Because he hoped that he could eventually find a way of liberating his mother, Perseus undertook the seemingly impossible task of slaying the Medusa. Using only the reflection of Medusa in his shield, he lopped off her head and put it in a sack. The Double Cluster traditionally represents the sword he used for that gruesome task.

Perseus had many adventures on the way home. Among other things, he rescued his future wife Andromeda from the clutches of the sea monster Cetus, but that’s another constellation story.

Polydectes was surprised to see Perseus return. He figured that the hero would not survive his encounter with the Gorgon. He was even more surprised, but only for a nanosecond, when Perseus pulled Medusa’s head out of the sack and turned him into a statue.

After many great adventures, Perseus returned with his mother and wife to Argos, the land of his birth. His grandfather Acrisius had been driven from power during their absence, and Perseus became king.

Despite his high status, he was a man of the people, often participating in their athletic competitions. At one such contest, he threw a discus with such force that he accidentally killed someone in the crowd. It was thus that the prophecy that began this tale came true. The unfortunate bystander was none other than Acrisius.

As for Perseus, he lives now among the stars, the head of Medusa with its single eye still clutched in his hand, his fuzzy sword still sheathed at his waist.

From his lofty perch, he speaks to us from our ancient past, when good deeds, long suffering, and a sympathetic heart never went unrewarded, at least in the stories they told.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.